Elucidation of the Al/Si Ordering in Gehlenite Ca2Al2SiO7 by Combined Si-29 and Al-27 NMR Spectroscopy/Quantum Chemical Calculations

作者：Florian Pierre^*; Veron Emmanuel; Green Timothy F G; Yates Jonathan R; Massiot Dominique

来源：Chemistry of Materials, 2012, 24(21): 4068-4079.

DOI：10.1021/cm3016935

• Al/Si ordering
• aluminosilicate minerals
• NMR spectroscopy
• first-principle calculations
• silicon-29
• aluminum-27

We have investigated the Al/Si ordering in the pseudoisolated pairs of tetrahedral sites of the structure of crystalline gehlenite Ca2Al2SiO7 by means of Si-29 and Al-27 NMR and first-principles quantum mechanical calculations. Si-29 NMR spectra of isotopically enriched samples enables the precise determination of the population of the two silicon sites Si-(OAl)(3-n)(OSi)(n) (n = 0, 1) and hence the amount of Al-O-Al linkages. This leads to a reliable and model-free quantification of the departure from the Loewenstein rule and to an experimental Al/Si ordering enthalpy of 50.4 +/- 1.6 kJ/mol fully reproduced by the quantum mechanical calculations. The seven aluminum sites arising from the Al/Si substitutions Al-(OAl)(4-p)(OSi)(p) (0 %26lt;= p %26lt;= 4) and Al-(OAl)(3-p)(OSi)(p) (p = 0, 1) are identified by Al-27 MAS, MQMAS, and {Si-29}Al-27 HMQC experiments, with their quantification being consistent with a fully disordered arrangement of the tetrahedral pairs in the a-b plane of the structure. Assignments of those strongly overlapping lines are further confirmed by density functional theory (DFT) calculations performed on a series of 2 x 25 supercells. An experimental and computational variation of -3 ppm of the Al-27 isotropic chemical shift is obtained for the substitution of one Al by one Si in the second coordination sphere of a central Al atom. Si-29 and Al-27 isotropic chemical shifts are seen to be sensitive primarily to short-range structural variations whereas a more complex behavior related to the nearby presence of Loewenstein-violating pairs is observed for the Al-27 quadrupolar coupling constant. Decomposition of the calculated EFG tensors into a sum of local, nonlocal, and ionic components demonstrates that it is almost entirely determined by the local electronic structure near the T-1 nucleus. The width of the distribution of NMR parameters is seen to strongly correlate to the degree of ordering present in the material. Scalar coupling constants (2)J(T-O-T) (with T = Al, Si) are found to be linearly related to the angle TOT bond angle.

• 出版日期2012-11-13